1. Field of the Invention
The present invention relates to a process for preparing a pure phosphoric acid from a crude aqueous phosphoric acid solution.
2. Description of the Prior Art
Crude phosphoric acid, prepared by decomposition of phosphate rock with sulfuric acid or hydrochloric acid, is purified using organic solvent-extraction methods or the like. In all these prior art processes, however, it is important to remove silica and flourine in order to prepare a phosphoric acid having a high degree of purity. The crude phosphoric acid usually contains relatively large amounts of these impurities depending upon the particular type of phosphate rock. Both a fluorine removal method and a silicofluoride recovery method for removing silicon and fluorine from the crude phosphoric acid have been proposed. However, these conventional methods have not proven to be satisfactory and the resulting purified phosphoric acid has not been of sufficient quality for use in foods.
A crude phosphoric acid typically contains 0.5 - 1% of Si and 2 - 3% of F. In the aforementioned silicofluoride recovering method, silica and fluorine are separated as precipitates of silicofluoride by addition of a precipitant such as sodium hydroxide, sodium carbonate, sodium chloride, potassium hydroxide, barium hydroxide or the like. This method is effective for treating crude phosphoric acid containing a large amount of silica and fluorine. However, an excess of the precipitant is required and the rate of removal of silicon and fluorine is only up to about 60%. It has been also disclosed to separate certain amounts of fluorine and silica by heating and condensing a crude phosphoric acid at high temperature (US Pat. No. 2,987,376). This method can be applied only to crude phosphoric acid containing relatively high amounts of silica and fluorine whereby a final concentration ratio of P/F, by weight, of about 100 - 200 is obtained. This is not satisfactory and, furthermore, the separation of silica is not disclosed.
The organic solvent-extraction method has also been proposed for removing impurities from a crude phosphoric acid. Certain impurities of silica and fluorine can be separated and removed by this method. However, the results are not satisfactory. The crude phosphoric acid usually contains an atomic ratio of F/Si of 4 - 6 as silicofluoride depending upon the kind of phosphate rock. When the crude phosphoric acid is treated by the organic solvent-extraction method, this combination of fluorine and silica is changed, depending upon the extraction process. The organic solvent-extraction method is usually conducted by counter-currently contacting a crude phosphoric acid with an organic solvent, such as an alcohol, e.g., butanol, in a series of extractors, e.g., mixer-settlers thereby repeating the extraction and separation, and then washing the extracted solution of the solvent and phosphoric acid and extracting phosphoric acid from it with water and the like. The dilute phosphoric acid obtained (from a counter-extractor) by the organic solvent-extraction method after contacting the extracted solution of the solvent and phosphoric acid with water (hereinafter referred to as the extracted aqueous phosphoric acid solution), contains fluorine and silica at an atomic ratio of F/Si of about 2 - 3. The fact of this decrease in the proportion of F has been found by the present inventors. The partition coefficients of silica and fluorine in the extraction of phosphoric acid (the ratio of the concentration in water to the concentration in the organic solvent) are usually 2 - 4 in the case of silica and 5 - 9 in the case of fluorine depending upon the nature of the organic solvent.
The inventors have found that silica precipitates as a scale of SiO.sub.2 in the heat tubes of the evaporator thereby causing a decrease in heat efficiency and increasing the likelihood of an accident clogging the pipes when a crude phosphoric acid is treated by the organic solvent-extraction method and the resulting extracted aqueous phosphoric acid solution discharged from the counter-extractor is condensed without further treatment. The phosphoric acid prepared by concentrating the extracted aqueous phosphoric acid solution has a concentration of Si of 50 - 200 p.p.m. and a concentration of F of 100 - 500 p.p.m.. These impurity levels are not satisfactory for use as a pure phosphoric acid. Furthermore, when phosphoric acid salts are prepared using the phosphoric acid, precipitations of colloidal silica are formed in the neutralized mother liquor and resulting difficulties of operation ensue. Consequently, there is a need for a technique of purification of crude phosphoric acid wherein silica and fluorine impurities are adequately removed.